1. Field of the Invention
The invention relates to a droplet ejecting device and, more particularly, to a droplet ejecting device which uses deformation of a piezoelectric transducer.
2. Description of Related Art
A piezoelectric ink jet type printer head has been conventionally proposed, wherein the volume of an ink passage is changed using the deformation of a piezoelectric transducer. Ink staying in the ink passage is ejected through an orifice at the time of a decrease in volume while ink is introduced into the ink passage, via a valve disposed on a side opposite to the orifice, at the time of an increase in volume. This type of ink jet printer head is called a drop-on-demand type. A plurality of ejectors, each structured as described are arranged adjacent to one another. The ink is ejected from the ejector(s) located in a predetermined position(s) so that a desired character or image is formed.
This type of droplet ejecting device is disclosed in, for example, U.S. Pat. Nos. 4,992,808; 5,003,679; 5,028,936. FIGS. 4 and 5 schematically show one of conventional droplet injecting devices. This conventional device will be explained in detail hereinafter referring to FIG. 4, which is a cross sectional view showing a part of an array of the conventional droplet ejecting device. A piezoelectric ceramic plate (piezoelectric transducer) 1, which has a plurality of side walls 2A, 2B, 2C and 2D and is polarized in the direction indicated by an arrow 51, is bonded to a cover plate 21 made of a metal, glass or ceramic material via a bonding layer 12. The walls 2A, 2B, 2C, 2D and the outside walls define ink passages 31A, 31B and 31C. Each ink passage 31 is formed into an elongated shape of a rectangular cross section. The side walls 2 extend along the entire length of the ink passage, and can be deformed in the vertical direction with respect to an axis of the ink passage and the polarizing direction. A metal electrode 11 for applying a driving electric field is formed on the side wall 2.
In the array, if the ink passage 31B is selected on the basis of a predetermined print data, a driving electric field is applied between the metal electrodes 11C and 11D, and between the metal electrodes 11E and 11F, respectively. Since the direction of the driving electric field is perpendicular to the polarizing direction, the side walls 2B and 2C are deformed inward of the ink passage 31B by a piezoelectric thickness shear effect. With this deformation, the volume of the ink passage 31B is decreased so that the ink pressure is increased. Accordingly, an ink droplet is ejected through an orifice 42 (see FIG. 5). When application of the drive electric field is stopped, the side walls return to their original positions, before the deformation, so that the ink pressure in the passage is decreased. Consequently, ink is supplied into the passage from an ink supplying portion (not shown).
The array is manufactured by the following method. As shown in FIG. 5, parallel grooves 3, constituting the ink passages having the above-mentioned shape, are formed in the piezoelectric ceramic plate 1, polarized in the direction indicated by the arrow 51, by grinding using a diamond cutting disk. On the sides of the groove 3, the aforementioned metal electrode is formed by spattering or the like. The cover plate 21 is bonded to the upper grooved surface 4A of the piezoelectric ceramic plate 1. An orifice plate 41 is bonded to the end surface 4B, on the ink ejecting side of the piezoelectric ceramic plate 1. The orifice plate 41 is provided with orifices 42 formed to correspond to the face of the ink passages.
In the above described conventional droplet ejecting device, the side walls of the piezoelectric ceramics are deformed inward of the ink passages by the piezoelectric thickness shear effect.
However, because the side walls of the piezoelectric ceramics are interposed between the adjacent ink passages, it is impossible to simultaneously eject ink droplets from the adjacent ink passages. Consequently, the array of the droplet ejecting device is divided into a plurality of groups for ejection control. Therefore, an ink ejecting cycle of the array as a whole in the droplet ejecting device is longer than that in the case where the ink droplets can be simultaneously ejected from the adjacent ink passages, with an attendant problem of a low print speed.
Furthermore, in the conventional droplet ejecting device described above, the metal electrode is disposed on the side walls, i.e., only on the inner surfaces of the groove. The metal electrode is disposed on the side walls and on the upper surface of the side wall by spattering or the like, and then, the metal electrode material disposed on the upper surface of the side wall must be removed. As a result, manufacturing of the metal electrode is complicated and difficult.